1. Field of the Invention
The present invention generally relates to variable magnification optical systems. Particularly, the present invention relates to a variable magnification optical system for increasing or decreasing the size of an image on an object plane, a projector using such a variable magnification optical system, and a reading apparatus.
2. Description of the Related Art
A projector is known that projects an image formed by an image forming unit onto a screen as an enlarged image. Desirably, the size of such a projector should be reduced as much as possible so that it can be installed as near the screen as possible. Japanese Laid-Open Patent Application Nos. 2008-096983 (“Patent Document 1”) and 2008-096984 (“Patent Document 2”) propose large-magnification optical systems which are entirely or partly (hybrid) constructed of an optical system including a mirror with a certain power. In both of these known systems, the mirror with a certain power is disposed in the final stage of a lens system (i.e., at a position closest to the screen along the optical path), and a very wide projected angle is realized by a high deflecting function of the mirror.
In the above optical systems according to Patent Documents 1 and 2, the maximum angle of view of more than 70° with respect to the screen can be obtained. Thus, depending on the design, projected magnification of more than 1.4 times can be obtained by changing a projection distance between the projector main body and the projected plane by as little as a few dozen centimeters. Namely, an image can be enlarged and projected onto a large screen at a very short distance, whereby the magnification ratio can be easily changed by a very small change in projection distance.
However, in order to provide such optical systems with a variable magnification function that does not require a change in the projection distance, a plurality of lens elements need to be moved. For example, Japanese Laid-Open Patent Application No. 2004-295107 (“Patent Document 3”) discloses an optical system in which a wide-tele zoom drive is performed by a variable magnification optical system.
In this optical system, an intermediate image is formed by a refracting system, reflected by a concave mirror, and then projected onto a screen as an enlarged image. The image formed by the refracting system is located closer to the screen than to the reflecting plane on which the image is next incident. In this optical system, plural lenses of which the refracting system is composed are moved parallel to the optical axis, thus forming a zooming optical system.
As an optical system in which only mirrors are used, Japanese Laid-Open Patent Application No. 2005-189768 (“Patent Document 4”) discloses a technology whereby a group of mirrors is moved for zooming and focusing. In this optical system, a light beam whose optical intensity is modulated by a light valve is projected onto a screen via plural curved reflective mirrors. The plural mirrors are moved in a coordinated manner to change the projected angle of view, thereby obtaining wide-angle-, middle-, and telephoto-images.
Japanese Laid-Open Patent Application No. 2006-184591 (“Patent Document 5”) discloses a technology for changing the magnification of a projected image by changing the projection distance L, without changing the projected angle of view. In this technology, the entire projector apparatus, including a projecting unit, optical systems, an LCD image generating unit, a light source lamp, and an illuminating optical system, is moved to change the projection distance.
In the optical system according to Patent Document 3, the concave mirror is fixed and a lens system of which the refracting system is composed is moved along the optical axis in order to provide a zooming optical system. Thus, the angle of view with respect to the screen is not constant, and the size of the concave mirror needs to be set for the wide-angle side. As a result, the size of the concave mirror increases.
In the optical system according to Patent Document 4, magnification is changed by changing the projected angle of view. Namely, the projected angle of view needs to be changed for zooming. Changing the projected angle of view results in an increase in the area of the final mirror, as in the case of Patent Document 3, because at the wide-angle end, the width of the projecting light beam is larger than at the telephoto end. Specifically, in order to accommodate wider-angle images, the size of the final mirror needs to be set for the wide-angle end, thus necessitating a larger-sized mirror.
A larger mirror requires greater precision during manufacture, resulting in an increased manufacturing cost for achieving a required level of performance. The greater size of the lens also results in a larger size of the system as a whole.
In the projector according to Patent Document 5, the projection distance cannot be changed unless the entire apparatus is lifted. The projector also requires a support unit strong enough to withstand the weight of the apparatus. When the apparatus is lifted, so is the center of gravity, thus making the apparatus unstable, which is not desirable when the projector is to be placed on top of a table or the like.